This invention relates to a radio frequency filter and in particular, to a filter constructed for printed circuit board or surface mount applications.
Conventionally, filters in the frequency range 300 MHz to several gigahertz are constructed using so-called quarter-wavelength resonators. Often these are of the so-called "engine block" variety which comprise a series of generally cylindrical transmission line resonators formed from metallized ceramic tubes of less than a quarter-wavelength electrical length, which are coupled to each other by their mutual proximity and via various arrangements of orifices and by removal of the metallization. Another common approach is to form a multi-layer device with inductor spirals formed on a dielectric substrate material coupled to multiple-layer capacitors. The latter approach suffers in particular from poor upper frequency performance limitation imposed by the spiral inductors due to unwanted fringing capacitances in the spirals.
Prior filter constructions in general are complicated to manufacture, requiring several different materials and processes to be used in their construction followed by complex mechanical processes such as lapping and re-plating to trim the filter to meet the required electrical specification. In addition, such filters are frequently the largest component on a circuit board and are not readily reduced in size. Such a reduction in size is desirable not least because this generally causes the spurious response inherent in such filters to move further up the frequency spectrum and thus further away from the spectral area of interest.